Ultrasonic power transfer using piezoelectric devices is a promising wireless power transfer technology for a wide variety of charging use cases, including the charging of mobile devices and biomedical implants. Ultrasonic transcutaneous energy transfer (UTET) devices (also known as “power links”) are a promising alternative to electromagnetic induction coils for providing power to devices across a medium such as water, air, or—as in the case of active implanted devices such as cochlear implants, pacemakers, and neurostimulators—through tissue. For many applications, UTET can provide power transfer efficiency and total power throughput comparable to magnetic induction coils, but in a much smaller and lighter device. Smaller power transfer devices could potentially benefit patients by providing an increase in the potential surgical sites for the implant unit, less surgical recessing, and increased comfort and aesthetic appeal for device users. Smaller devices may also use smaller alignment magnets, which may increase MRI compatibility for the implant.
However, power transfer efficiency (PTE) tests reveal a challenge in the design of UTET systems, as efficiency levels show extreme sensitivity to the transmitting frequency and the acoustic separation distance between the transmitting and receiving piezoelectric transducers. PTE sensitivity is a result of acoustic reflections at the transmit and receive transducers. These reflections create persistent acoustic energy in the tissue in the form of a standing wave between the transducers at steady state. The standing wave can have a very significant impact on the acoustic impedance looking into the acoustic cavity formed between the transducers and, at specific frequencies, can result in improved impedance matching between the transducer and tissue by making the cavity appear less stiff to the transducer than it would be in the absence of the standing wave. At other frequencies the standing wave can worsen the impedance match by making the cavity appear stiffer to the transmit transducer. This frequency variation is the source of the frequency dependence of the PTE and impedance characterized by minima and maxima that are periodic with frequency.
In a subdermal implant, the acoustic separation distance between the external and implanted parts of the power link is expected to vary considerably between patients, and will also change with patient movement, hydration, and tissue growth. This sensitivity cause large swings in efficiency and represents a serious limitation on battery life and overall device reliability. Thus, techniques are needed to either passively reduce the dependence of transfer efficiency on separation or to actively compensate for changes in separation distance. There has not yet been a reported UTET system that performs real time compensation for separation distance changes in order to maximize power transfer efficiency.